A MOS transistor is widely used as switching devices. In contrast to conventional MOS transistors containing a gate electrode which is formed of poly silicon, a metal material with superior electric conductivity better than the poly silicon have been used as the gate electrode of MOS transistors. MOS transistors are classified as n-MOS transistors or p-MOS transistors in accordance with the channel type which is induced beneath the gate electrode. The gate electrodes of the n-MOS transistor and the p-MOS transistor may be formed of different metals so that the n-MOS transistor and the p-MOS transistor have different threshold voltages.